We have performed cross-gain modulation experiments on semiconductor optical amplifiers with 100fs pump pulses. Setting the pump on the high energy side of the gain results in a complex transient for the CW probe that is situated at gain maximum. An important part of this transient is shown to be due to propagation effects. Indeed, the pump pulses, at transparency or in absorption at the entrance, may get out of the amplifier in the gain region. These effects have to be considered, particularly in the potential sub-picosecond wavelength conversion with 100fs pulses placed at transparency. Simulations based on a microscopic theory, which goes beyond the relaxation time approximation, show that, in such conditions, electron and hole populations can have different temperatures and different evolutions. However these simulations, which neglect propagation effects and consider only one slice of the device, fail to explain some of the experimental curves. In very long amplifiers (more than 2mm) propagation effects become even more important. We show that they can be distinguished from carrier heating due to the generation of high energy carriers that arises only above an energy per pulse of 40pJ.

We propose a new configuration for semiconductor optical amplifiers (SOAs), called optical speed-up at transparency (OSAT), which allows to speed up the gain recovery of SOAs and their saturation power without sacrificing the gain, nor increasing the applied current, The proposed configuration is particularly well-suited for high-speed WDM or OTDM applications, It consists of an optical cw-signal injected at the transparency point of the SOA, This setup is potentially integrable on a single chip, and still relatively flexible once the device is realized.

Summary form only given.Although semiconductor optical amplifiers (SOA) are required for all-optical devices and are to be used in future telecommunication networks, little is known about their internal carrier density and temperature distributions and their dependencies on various parameters such as applied current, injected optical signal power, and active layer geometry. Measurements of spontaneous emission (SE) provide information on both density and temperature of the carriers. Such an experiment has already revealed longitudinal spatial hole burning (LSHB) and carrier heating in an SOA working at 1.55 /spl mu/m. LSHB in this structure is caused by amplified spontaneous emission (ASE) and by injection of an optical signal. Here, we present for the first time to our knowledge, spatially resolved temperature measurements on SOAs.

Summary form only given. High-speed semiconductor optical amplifiers (SOAs) are required in many devices for all-optical processing in WDM networks. They can be used as linear devices (optical gates for optical switching for example) or they can be required for their nonlinear properties (to realize complex functions like wavelength conversion). The extremely fast gain recovering in SOAs is required for high-bit-rate applications in future optical systems. The gain recovery time can be only moderately reduced by increasing the conventional SOA length but this also reduces the optical gain bandwidth. The SOA setup proposed in the present paper, called OSAT (optical speed-up at transparency), uses an assist light beam injected at the transparency point of the SOA, which gives rise to higher possible gain and higher speed at a much lower injection current while eliminating relaxation oscillations and dark holes. The principle of the OSAT is based on the carrier dependency of the material gain transparency. We have demonstrated that a number of features makes it very attractive for all-optical processing in WDM networks.

We have measured the ultrafast simultaneous cross-gain and laser mode dynamics in a gain-clamped semiconductor amplifier perturbed by an intense detuned 150 fs pump pulse. Besides relaxation oscillations, we demonstrate the instantaneous formation of a dark pulse in the laser mode that repeats itself with a period given by the cavity round-trip time. The dark pulse sequence subsequently decays into two-mode beating and is shown to weakly cross modulate the amplifier gain. To describe dark-pulse formation a time- and spatially dependent model based on rate equations is necessary. The experimental results are in reasonable agreement with numerical simulations.

Measurements of spontaneous emission from InGaAsP semiconductor optical amplifiers provide information on both the carrier density and temperature. By spatially resolving the light emitted along the active layer of the device, we find evidence of longitudinal spatial hole burning which results from amplified spontaneous emission in the structure and is modified by the injected optical signal. Under injection, we also observe pronounced asymmetry of the amplified spontaneous emission intensity from the two facets which we relate to the carrier density profile. The experimental results are in good agreement with numerical simulations. An analysis of the measured spectra reveals an unexpected very high temperature (400 K) and its decrease by at least 35 K in the middle of the device when light is injected.

We have performed ultrafast cross gain modulation experiments on a gain clamped semiconductor amplifier. Time-resolved observation of the lasing mode intensity after excitation with a strong, detuned pump pulse of 150 fs duration reveals the instantaneous formation of a dark pulse that repeats itself with a period given by the cavity round- trip time. A simultaneously measured cw probe signal at gain maximum shows pronounced relaxation oscillations. The experimental results are in reasonable agreement with numerical simulations.

Photoluminescence lifetimes of the n = 2 level in a large quantum well show a clear nonmonotonic dependence on the density of optically generated carriers. Varying the power density over five orders of magnitude we prove directly the high efficiency of carrier-carrier interaction for intersubband scattering when longitudinal-optical phonon emission is suppressed. For low densities, the observed n = 2 decay times get shorter (from 40 down to 5 ps) as the density is increased. At high densities Pauli blocking reduces significantly the intersubband scattering rates. [S0163-1829(99)06124-X].

Summary form only given. Ultrafast gain dynamics in semiconductor optical amplifiers (SOAs) has recently received considerable interest because of the potential application of SOAs in high-speed optical networks. It also provides a better understanding of the fundamental physical processes involved in the interaction between a short optical pulse and a high density plasma. We have performed cross-gain modulation experiments with 120 fs time resolution on a 600 ym long SOA. A weak cw probe beam is injected into the SOA, together with a strong pump pulse. The probe output signal is time-resolved by upconversion.

Gain-clamped semiconductor optical amplifiers (GCSOA’s) are promising candidates for wavelength division multiplexing (WDM) applications due to their low inter-channel crosstalk and high saturation output power, obtained by the gain-clamping [1]. However, in this presentation we will show that, although the gain is clamped, some inter-channel crosstalk still occurs due to longitudinal spatial hole burning.

Ultrafast gain dynamics in semiconductor opticel amplifiers (SOA's) is of great interest because of the potential application of SOA's in high-speed optical networks. It also enables to obtain a better understanding of the fundamental physical praccsses involved in the interaction between a short optical pulse and a high density plasma. Here, we present experimental results obtained by a 150 fs CmSgain modulation technique. This novel technique enables lis to probe the gain recovery at different wavelengths arouud tile pump wavelength with a high spectral resolution. The pump wavelength itself can be varied over B large region. We will present results obtained with the pump io the gain region as well M in the absorption region. The results are obtained on a 500 pm long SOA operated a t 250 m 4 , providing a maximum gain of 29 dB a t 1530 urn. The experimental results show complex sub-picosecond gain dynamics, attributed to different physical pmcesses, such BS two-photon absorption, spectral hole burning and carrier heating. From these results the different time constants for these processes are extracted. Their dependence on both pump arid probe wavelength is inveatigated. By changing the probe wavelength whiie keeping the pump wavelength fixed. we are able to obtain the dependence of C lan aud agj8T on wavelength. The experimental results arc compared with a detailed theoretical analysis based on the numerical solution of the semiconductor Bloch equatioos. Apart from coherent manybody effects the inodel contains ali carrier-carrier scattering trim8 up to second order (Born approximation) that contribute to the thermalization of the particle distribution functions as well as to polarization transfer and decay. The use of microscopic collision integrals iiiscead af phenomenological relaration and dephasing times is essentiai to obtain agreement between theo re t id and expcrimmtal results.

A dynamical theory of multiphoton transitions in semiconductors is developed using a density matrix approach. It is shown that, in dipole approximation, these transitions require a mixed parity of the basis stales and that band-diagonal coupling's of the electromagnetic field to Bloch electrons have to be included. The general theory is outlined using the simple, but solvable model of a diatomic tight-binding chain. Possible extensions and applications of the approach are discussed.

We have evidenced co- and counter-propagating dark pulses in the laser mode of gain clamped semiconductor optical amplifiers (GCSOA)s perturbed by an intense ultrafast pulse. They decay into a long-lived two mode beating. The resulting gain modulation may be a limiting factor for applications.

Ultrafast gain dynamics in semiconductor optical amplifiers (SOA’s) is of great interest because of the potential application of SOA’s in high-speed optical networks. It also enables to get a better understanding of the fundamental physical processes involved in the interaction between a short optical pulse and a high density plasma. So far, the ultrafast gain dynamics in SOA’s has been mainly studied by means of standard pump and probe (P&P) measurements in which the wavelengths of the pump and probe are identical. Here, we present a new experimental technique which is an ultrafast cross-gain modulation (XGM) between a 150 fs pump pulse and a weak cw probe signal. The advantages of the XGM over the P&P technique are two-fold. Firstly, the probe wavelength can be varied and therefore the gain recovery at different wavelengths around the pump wavelength can be obtained. Secondly, the probe signal is a cw signal, thus having a very narrow linewidth and hence a high spectral resolution. Consequently, information is obtained on the spectral carrier population distribution and its recovery.

Using femtosecond resonant luminescence, we have measured the intersubband scattering fare of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing between the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carrier-carrier interaction is responsible for the ultrafast transitions.

The longitudinal spatial hole burning (LSHB) in gain-clamped semiconductor optical amplifiers (GCSOAs) is investigated by means of a numerical model, which is based on position-dependent rate equations for the carrier density and the propagation equations for the optical power. The simulation results show that the carrier densities are nonuniformly distributed within the active layer of GCSOAs. The nonuniformity can be large, especially for high currents and optical signal powers near the saturation. It is found that the LSHB induces a gain nonlinearity, which causes interchannel cross talk when GCSOAs are used in wavelength division multiplexing (WDM) applications. In order to reduce this gain nonlinearity, two methods are analyzed: the use of low resistivity devices and the use of unbalanced Bragg mirror reflectivities

Pump and probe measurements with femtosecond resolution are performed on a gain-clamped semiconductor optical amplifier, a structure that contains a lasing mode. The corresponding relaxation oscillations are observed in the temporal gain recovery of the amplifier. The current dependence of the oscillations is well reproduced by a small-signal analysis of the laser rate equations. The gain recovery can be very fast, we measure damping rates of up to 40 GHz.

This paper shows experimental data and simulation results of the gain recovery of gain-clamped semiconductor optical amplifiers (GC-SOA). Damped relaxation oscillations are observed. Both the oscillation frequency and damping rate increase with applied current and can attain very high values. We measured a gain recovery time of 26 ps at 200 mA applied current and an oscillation frequency of 11 GHz. A good agreement between experimental data and simulation results is obtained. Simulation results indicate that a GC-SOA used as amplifier in wavelength division multiplexing (WDM) networks can amplify signals at very high bit-rates with low cross-talk and extinction ratio compression.

Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing be tween the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carrier-carrier interaction is responsible for the ultrafast transitions.